Organic electroluminescent is a light-emitting phenomenon that organic materials directly convert electric energy into light energy under the effect of electric field. In early days, due to high driving voltage and low luminous efficiency of device prepared, research of organic electroluminescent was being held back. Until 1987, Tang and Van Slyke of Eastman Kodak invented thin uniform compact films of high quality using tris (8-hydroxyquinoline) aluminum (Alq3) as a luminescent material together with aromatic diamine, and then prepared organic electroluminescent device of higher brightness and efficiency at relatively lower working voltage, thus started a new stage of the research of organic electroluminescent material. Theoretically, the internal quantum efficiency of fluorescent materials is merely 25%, limited by Spin Statistics theory, how to take advantage of the rest 75% brought by phosphorescent emission to obtain higher luminous efficiency has been becoming a research focus in the field. In 1997, the phosphorescent electroluminescent phenomenon was discovered by Forrest et al., after which the limit of 25% internal quantum efficiency of organic electroluminescent material was broken and the organic electroluminescent material field embraced another new stage.
In current research on organic electroluminescent materials, small molecules of doping transition metal complexes, such as iridium, ruthenium and platinum complexes have been actively researched. The advantage of the complexes is that they can obtain high emission energy from their triplet states, wherein the Ir(III) compounds have been taking a leading position in the research followed ever since for characteristics of good stability, mild synthesis reaction condition and high electroluminescent capacity. To get a full-color electroluminescent display, efficient red, green and blue electroluminescent materials with excellent performance should be available at the same time. Compared with red and green, blue electroluminescent materials are less developed, consequently, growing interests has been focused on how to improve the luminous efficiency and color saturation of blue electroluminescent materials. Iridium(III) bis[2-(4′,6′-difluorophenyl)pyridinato-N,C2]picolinate (Flrpic), an Ir(III) metal organic complex, is one of the most reported blue electroluminescent materials. The luminous efficiency and color saturation of the electroluminescent materials such as Firpic and alike have been greatly improved by ligand modification and device optimization. In 2005, Holmes, Forrest et al. firstly synthesized Iridium(III) tris(1-phenyl-3-methylbenzimidazolin-2-ylidene-C,C2′), abbreviated as Ir(pmb)3, an organic electroluminescent material carrying 1-phenyl-3-methylbenzimidazolyl as the ligand, of which the CIE coordinates were (0.17, 0.06) (Applied Physics Letters. 2005, 87, 243507). Generally, 2-picolinate serves as a prior ancillary ligand for blue metal Ir(III) organic electroluminescent material, such as Firpic and iridium(III) bis{4-phenyl-2-[6-(N-(2-(2-methoxyl) oxethyl)ethyl)carbazolyl]quinoline}(2-picolinate) ((EO-CVz-PhQ)2Ir(pic)) and etc. (Adv. Funct. Mater. 2009, 19, 2205-2212). Further more, complexes with ligands substituted by fluoro group demonstrate a greater hypochromatic shift effect than corresponding complexes without ligands, for example: Iridium(III) tris[2-(4′,6′-difluorophenyl)pyridinato] (Ir(F2ppy)3) exhibits a hypochromatic shift by 37 nm under the temperature of 77K and by 42 nm under 298K than iridium (III) tris(2-phenylpyridinato) (Ir(ppy)3); iridium (III) bis[1-(4′,6′-difluorophenyl)pyrazolyl](2-phenylpyridin) ((F2ppz)2Ir(ppy)) exhibits a hypochromatic shift by 14 nm under the temperature of 77K and by 25 nm under 298K than iridium (III) bis(1-phenylpyrazolyl)(2-phenylpyridinato) ((ppz)2Ir(ppy)) (J. AM. CHEM. SOC. 2009, 131, 9813-9822).